NAME

       io_uring_prep_recv - prepare a recv request

SYNOPSIS

       #include <liburing.h>

       void io_uring_prep_recv(struct io_uring_sqe *sqe,
                               int sockfd,
                               void *buf,
                               size_t len,
                               int flags);

       void io_uring_prep_recv_multishot(struct io_uring_sqe *sqe,
                                         int sockfd,
                                         void *buf,
                                         size_t len,
                                         int flags);

DESCRIPTION

       The io_uring_prep_recv(3) function prepares a recv request. The submission queue entry sqe
       is setup to use  the  file  descriptor  sockfd  to  start  receiving  the  data  into  the
       destination buffer buf of size len and with modifier flags flags.

       This  function  prepares  an  async  recv(2) request. See that man page for details on the
       arguments specified to this prep helper.

       The multishot version allows the application to issue  a  single  receive  request,  which
       repeatedly  posts  a  CQE  when  data  is  available.  It  requires  length  to  be 0, the
       IOSQE_BUFFER_SELECT flag to be set and no MSG_WAITALL flag to be set.  Therefore each  CQE
       will  take  a  buffer out of a provided buffer pool for receiving.  The application should
       check the flags of each CQE, regardless of its result.  If a posted CQE does not have  the
       IORING_CQE_F_MORE  flag  set  then  the multishot receive will be done and the application
       should issue a new request.  Multishot variants are available since kernel 6.0.

       After calling this function, additional io_uring internal modifier flags may be set in the
       SQE ioprio field. The following flags are supported:

       IORING_RECVSEND_POLL_FIRST
              If  set,  io_uring  will  assume  the  socket  is currently empty and attempting to
              receive data will be unsuccessful. For this case, io_uring will arm  internal  poll
              and  trigger  a  receive  of  the  data  when the socket has data to be read.  This
              initial receive attempt can be wasteful for the case where the socket  is  expected
              to  be  empty,  setting  this  flag  will bypass the initial receive attempt and go
              straight to arming poll. If poll does indicate that data is ready to  be  received,
              the operation will proceed.

              Can  be  used with the CQE IORING_CQE_F_SOCK_NONEMPTY flag, which io_uring will set
              on CQEs after a recv(2) or recvmsg(2) operation. If set, the socket still had  data
              to  be  read  after  the  operation completed. Both these flags are available since
              5.19.

RETURN VALUE

       None

ERRORS

       The CQE res field will contain the result of the operation. See the related man  page  for
       details  on  possible  values.  Note that where synchronous system calls will return -1 on
       failure and set errno to the actual error value, io_uring never uses  errno.   Instead  it
       returns the negated errno directly in the CQE res field.

SEE ALSO

       io_uring_get_sqe(3), io_uring_submit(3), recv(2)